(1) Field of the Invention
The present invention relates to a microwave fryer using radiated energy of microwaves, and particularly to a microwave fryer in which oil in and oil vessel, detachably provided in a heating chamber, is heated by microwaves. More specifically, the present invention is directed to a microwave fryer which is capable of distinguishing the propriety of the quantity of oil held in an oil vessel, or prevents the user from forgetting the use and forgetting to turn it off as well as to effect automatic cooking, and which has a temperature detector detecting a real temperature of the cooking oil.
(2) Description of the Prior Art
Japanese Patent Publication Hei-1 No.26,689 has disclosed a fryer.
This conventional fryer has a heating chamber in the main apparatus body, an oil vessel storing oil beneath the heating chamber with a zigzag heater near the bottom of the oil vessel and a microwave generator generating microwaves in the main apparatus body.
Provided in the heating chamber is a basket-like container holding foodstuffs, which can be moved up and down by a lifting mechanism.
Another conventional fryer has been disclosed in Japanese Patent Application Laid-Open Sho-59 No. 62,024.
This fryer comprises a sheathed heater for heating oil inside a heating chamber in the main body. Oil in the heating chamber is circulated between the heating chamber and an oil vessel provided separately from the heating chamber by means of a circulating pump. A microwave Generator is provided which impresses microwaves to foodstuffs held in the oil vessel as well as the oil circulated inside the oil vessel by the circulating pump.
Since the conventional fryer has the sheathed heater for heating the oil inside the oil vessel, it suffers from a performance problem that the oil is subject to deteriorate due to the contact of the oil with the sheathed heater at high-temperatures. Further, it is difficult to clean the oil vessel since the sheathed heater is disposed in the oil vessel.
In the aforementioned fryer disclosed in Japanese Patent Publication Hei-1 No. 26,689, there is a fear that, if microwaves are applied from the microwave generator when the basket-like container holding some foodstuffs is stationed at an upper position by the lifting mechanism, the foodstuffs are directly heated by the microwaves and cooked before being cooked by the oil.
The fryers described hereinabove effect an automatic frying operation, but the fryer of this kind cannot effect satisfactory frying if the oil is low in quantity and little in depth. On the contrary, if the oil it too large in quantity and great in depth, the oil runs over the top. Japanese Patent Publication Sho-60 No. 28,494 discloses a fryer in which a pair of temperature detectors for monitoring oil-temperatures are provided inside an oil vessel in order to detect the oil quantity inside the oil vessel based on the temperature difference between the temperatures detected by the two temperature detectors.
The oil vessel is fixed in any of the aforementioned fryers and it is impossible to apply the oil level detecting device disclosed in Japanese Patent Publication Sho-60 No. 28,494 stated above to a fryer having a detachable oil vessel. The necessity of two temperature detectors in the oil level detecting device also presents a disadvantage in cost.
In order to bring the aforementioned fryer into market and obtain certain consumer needs, it is preferable to distribute it as a multi-functional product having functions such as rice cooking and team cooking functions, other than the frying function. To meet this demand, it is necessary to propose an oil vessel detaching structure which facilitates an oil vessel easily replaced with a water vessel by anyone.
In order to avoid the problem of oil scattering at times of putting in or taking out foodstuffs, it is preferable for a home-use fryer to have such a structure disclosed as in Japanese Patent Application Laid-Open Sho-59 No. 62,024, that is, the structure in which the cooked stuff inside the oil vessel is fried with heat from the heated oil supplied and with heat generated by microwaves. The fryer disclosed in Japanese Patent Application Laid- Open Sho-59 No. 62,024, however, requires an oil vessel independent from the heating chamber and therefore not only becomes bulky but also requires a circulating pump for circulating the oil as well as circulating paths for the oil, becoming complicated in the whole structure.
In order to take measures against a case where the user goes away from the site without cutting the power supply after heating is started, another conventional fryer is disclosed in Japanese Patent Application Laid-Open Hei-4 No. 20,310, which, for example, has a device for monitoring the temperature of oil with a thermosensitive element and automatically shutting down the power supply to stop heating if any sharp lowering of the temperature of oil is not detected within a predetermined period of time.
As to the automation of cooking fried foods, there have been a lot of methods disclosed in which heating time is controlled in accordance with Variations of the oil temperature during cooking from a preset temperature in order to regulate the cooking of the resulting foodstuffs (for instance, Japanese Patent Publication Hei-5 No. 25,487, Japanese Patent Publication Sho-57 No. 54,127, Japanese Patent Application Laid-Open Sho-62 No. 32,926 etc.).
Since the prior art fryers of this kind use heaters and the like as a heating source, it is possible to regulate the oil temperature all the time and the user can monitor the used condition by checking the variation of the oil temperature. On the other hand, the high-frequency wave fryer utilizing high-frequency wave heating must necessarily be operated with its the heating chamber closed and with the door closed during heating. Conversely, when the door is open, the apparatus must stop heating. Accordingly, if the user goes away from the site leaving the door open for example, the temperature of oil lowers. In this situation, if the frying operation is started, the cooking of foods could be degraded.
In the conventional automatic fryer, a standard quantity of heat to be imparted to a predetermined quantity of foodstuffs is set up in advance and heating time is adjusted in order to compensate variations of heating quantity due to the varying oil temperature. This method is effective when an approximate amount of foodstuffs to be cooked is decided. In home use, however, the amount of foodstuffs to be cooked at the same time varies largely. Therefore, recognition of the amount of foodstuffs has a great influence on the cooking of the food. That is, in some cases, the estimate of the standard quantity of heat itself may pose a problem in determining a proper heating-time.
On the other hand, as to the fryer for frying foodstuffs, Japanese Patent Publication Hei-2 No. 60,323 discloses a fryer in which two different heating sources are provided in order to shorten the frying time. More specifically, a heater is provided which directly heats the cooking oil to allow the oil to braise the food from the outside while a high-frequency (microwave) heating device excites the inside of foodstuffs. That is, in general, the dielectric loss of foodstuffs is so great as compared to that of oils that when microwaves are applied to a system where foodstuffs and oil coexist, energy of microwaves is largely absorbed by the foodstuffs. Thus the foodstuffs are heated from the inside with the microwaves as well as braised in oil from the outside. The conventional temperature detector Used for detecting the temperature of oil is merely composed of a bar-shaped thermosensitive element such as a thermistor etc., covered with a metallic protecting tube. This protecting tube is provided in order to protect the thermosensitive element from the cooking oil as well as to accelerate the response of the sensor to changes in temperature.
However, since the conventional fryer has the two different heat sources, i.e., of radiation of microwaves and of conduction of heat generated by a gas burner, a heater or the like, the apparatus becomes large-scale, posing demerits such as high-cost for private use and taking up much room for installation. The temperature detector provided in the conventional fryer is designed considering only the protection against the cooking oil and the responding performance to changes in temperature. That is, since the conventional temperature detector is of a metallic bar as stated above, if this temperature detector is applied to a fryer in which cooking oil is heated by only radiation of microwaves, high-frequency electric fields concentrate on the tip of the temperature detector whereby the temperature detector itself is heated, thereby making it difficult to measure the real oil-temperature.